The supercooled state of molecular liquids reveals a mosaic structure

25-03-2021

Researchers investigating the microscopic origin of glass formation have used the ESRF’s nuclear resonance beamline ID18 to probe the molecular motions in a supercooled liquid near its glass-transition temperature. The results provide strong experimental support for theories of an intermittent mosaic structure in the deeply supercooled liquid phase.

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A glass can be thought of as a liquid that is no longer able to flow [1]. This definition reflects the typical procedure used to produce glasses, that is by quenching from the melt fast enough to prevent crystallisation. When a liquid is cooled down below its melting temperature, its viscosity becomes larger and larger and its molecular dynamics slows down to the point that when the glass-transition temperature (Tg) is reached, the liquid is so viscous that it appears frozen on the typical observation timescale: a glass has been obtained. Glasses are used in many technological applications, yet the microscopic origin of the dynamic arrest occurring at the glass transition is still debated and remains one of the most intriguing open questions in the field of condensed matter physics. 

A possible route to investigate this phenomenon is to study the molecular motions, called relaxation processes, through which a liquid can restore equilibrium following an external perturbation or a spontaneous fluctuation. Relaxations in a supercooled liquid are complex, heterogeneous processes, spanning many orders of magnitude in time. For instance, in a liquid above its melting temperature, the local structure changes on a timescale of a few picoseconds, while close to the glassy state, where the dynamic arrest occurs, the structural relaxation is of the order of 100 seconds. This structural relaxation is, however, anticipated by the appearance of a faster process of molecular re-arrangements known as the Johari-Goldstein (βJG) relaxation [2], which remains active even below Tg and whose role in the glass transition – whether that of an actor or of a spectator – is still debated. Experimental difficulties in accessing the spatial and temporal range characteristic of this process, i.e., interatomic length-scales and times of the order of hundreds of nanoseconds, means this question has been left open.  

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Fig. 1: a) Sketch of the typical experimental setup for TDI. The red lines indicate the direction of the incident and scattered X-ray beam. The black dashed line shows the direction of the transmitted beam. b) Example of time-domain interferometry data (squares with error bars) as a function of time in the supercooled state of 1-propanol.


Researchers from the University of Trento and the University of Pisa in Italy, and the Polytechnic University of Catalonia in Spain have used time-domain X-ray interferometry (TDI) at the ESRF’s nuclear resonance beamline ID18 to probe and characterise the dynamical properties of the Johari-Goldstein relaxation in the glass-forming liquid 1-propanol close to its glass-transition temperature. TDI experiments are designed to detect X-ray scattering from the sample using an interferometer (Figure 1), providing information that is both time and space resolved and allowing researchers to probe molecular motions at the timescale where the βJG process takes place.

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Fig. 2: Sketch of the molecules undergoing the βJG relaxation (red spheres) at a given time. They are highly mobile, perform larger spatial excursions than the rest of the molecules (white spheres) and are spatially connected in a percolating cluster.      


Combining their data with the literature, the researchers were able to provide a new picture for the microscopic dynamics in the supercooled liquid state. The molecules participating in the Johari-Goldstein relaxation form a percolating cluster that spans the whole sample (Figure 2). The βJG process thus marks the development of a mosaic state in the supercooled liquid phase characterised by patches of less mobile molecules that are separated by an ever-changing network of more mobile ones. These observations provide new insights into the heterogeneous dynamics of glasses. The appearance of such an infinite cluster, related to the βJG process, supports the idea that a dynamical transition is taking place in the supercooled liquid phase at low enough temperatures, as suggested by some theoretical models of the glass transition [4].


Principal publication and authors
Experimental evidence of mosaic structure in strongly supercooled molecular liquids, F. Caporaletti (a,g) S. Capaccioli (b,c), S. Valenti (d), M. Mikolasek (e), A.I. Chumakov (e,f), G. Monaco (a,h), Nat. Commun. (2021); https://doi.org/10.1038/s41467-021-22154-8
(a) Dipartimento di Fisica, Università di Trento, Povo (Italy) 
(b) Dipartimento di Fisica, Università di Pisa (Italy) 
(c) CISUP, Centro per l’Integrazione della Strumentazione dell’, Universitá di Pisa (Italy) 
(d) Department of Physics, Universitat Politécnica de Catalunya, Barcelona (Spain) 
(e) ESRF
(f) National Research Center ‘Kurchatov Institute’, Moscow (Russia)
(g) Present address: Van der Waals-Zeeman Institute, Institute of Physics/Van’t Hoff Institute for Molecular Sciences, University of Amsterdam (Netherlands)
(h) Present address: Dipartimento di Fisica ed Astronomia, Universitá di Padova (Italy)

References
[1] C.A. Angell, Science 267, 1924 (1995).
[2] G.P. Johari & M. Goldstein, J. Chem. Phys. 53, 2372 (1970). 
[3] A.Q.R Baron et al., Phys. Rev. Lett. 79, 2823 (1997).
[4] J.D. Stevenson & P.G. Wolynes, Nat. Phys. 6, 62-68 (2010).